This invention relates to molecular sieves that exhibit low inherent thermal stability, whereby removal of water or their organic template therefrom during calcination results in a collapse of their framework structure. More particularly, the invention relates to a method for overcoming the low stability of certain of these materials via a solid state reaction of such material with a salt.
Zeolites, both natural and synthetic, are microporous crystalline aluminosilicate materials of variable composition and are characterized by a 3-dimensional structure that contains channels and/or cages of molecular dimensions. Zeolites are differentiated from each other by their composition and structure. Most zeolites are assigned structure type codes of three letters, e.g., FAU for zeolite X, GME for gmelinite, and BPH for Linde Q. Characteristics, composition and structure determine the physical and chemical properties of each zeolite and their application to different industrial processes. An organic additive (e.g. tetraalkylammonium cation), commonly called a template, is needed for the formation of some molecular sieves.
The thermal stability of molecular sieve frameworks varies considerably, with many materials being unable to retain their framework intact once water and/or its organic template is removed during calcination.
Thus while a large number of aluminosilicate zeolites may be reversibly dehydrated with only minor distortions of the framework, others are dependent on occluded water molecules in order to retain their framework.
The reasons for the instability of these molecular sieves during calcination are not completely understood. The following are considered as the main factors: (1) template, hydrated cation or water molecules stabilize the high energy structural units rings or cages; and thus removal of these from the structure during calcination results in structural collapse (2) protons formed by decomposing organic templates destroy the framework structures, particularly for aluminum-rich zeolites; (3) cations, which balance framework negative charges, occupy wrong locations or have a wrong size or charge density to stabilize the building units in dehydrated frameworks.
The following are examples of zeolites that require improved thermal stability during calcination in order for them to be suitable for practical applications. Gmelinite (GME) is a well-known aluminosilicate zeolite, having a structure in which the main feature is a large 12-ring channel. Gmelinite (GME) exists naturally as a mineral, and can also be synthesized from an all inorganic mixture in the laboratory. Both natural and synthetic gmelinites behave like small-pore zeolites due to the intergrowth with chabazite, another zeolite, or structure faults. Chabazite-free gmelinite has been synthesized using a polymeric template synthesis system (see U.S. Pat. No. 4,061,717 and Daniels et al., JACS, 3097, 1978). Gmelinite (GME) prepared according to the Daniels et al. method undergoes degradation and loss of crystal quality on calcination.
Linde Q (BPH) was first synthesized in 1961 (see U.S. Pat. No. 2,991,151) and has been designated zeolite K-I and zeolite Q. Zeolite K-I can be made using kaolinite as source to give a composition of K2O:Al2O3:2SiO2:4H2O. In situ monitoring by X-ray diffraction reveals that K-I decomposes at 168xc2x0 C. before loss of all zeolitic water.
Linde Q shows major structural collapse on dehydration at temperatures as low as 240xc2x0 C. This degradation has severely limited possible applications and is unusual among aluminosilicate zeolites, most of which are stable to dehydration. Linde Q would have useful catalytic and sorption properties, if it could maintain its framework structure during calcination.
The most common strategy for increasing the thermal stability of thermally unstable zeolites (e.g. those zeolites unable to retain their framework during calcination) is to increase the framework Si/Al ratio. This can achieved by the following methods or combinations thereof: (1) steam treatment at high temperature resulting in dealumination of the framework (e.g., U.S. Pat. No. 4,724,067); (2) treating the zeolite with an organic or inorganic acid to dissolve some framework aluminum (e.g., U.S. Pat. Nos. 4,724,067; 4,909,924; 5,389,357); (3) treatment of the zeolite with a solution of a fluorine compound (e.g., U.S. Pat. No. 3,933,983); (4) treating the zeolite with a silicon-containing compound to replace aluminum with silicon (e.g., U.S. Pat. Nos. 5,389,357 and 5,236,877); and (5) treating the zeolite with a dealuminating agent, such as, EDTA.
U.S. Pat. No. 5,277,793 provides an improved hydrocracking catalyst Y-type which comprises less than about 0.5 wt. % alkaline metal oxide and which contains an effective amount of oxometallic cations positioned in the beta-cages of the zeolite. This substantially stabilizes the zeolite against thermal degradation and is a special case for improving the thermal stability of zeolite Y. Another effective method for increasing the thermal stability of crystalline zeolites is by introduction of rare earth cations into the pores or cages of the zeolite. In U.S. Pat. No. 4,701,431, a rare earth stabilized aluminum deficient zeolite having the structure of faujasite is provided.
Ion exchanging the zeolite with a specified amount of rare earth metal cations also stabilizes the aluminum deficient zeolite. U.S. Pat. No. 5,382,420 shows that a rare-earth exchanged form of zeolite Q has high thermal stability. However, some of the rare earth ions are not removeable, and a minimum quantity must be maintained to prevent collapse of their framework structure.
Unfortunately, for those materials that would be useful as N2 selective zeolite adsorbents the above methods are generally undesirable. This is because such zeolites preferably would include lithium and have a low Si/Al ratio.
It is therefore an object of the invention to stabilize structures that were previously thought to be unstable.
It is another object of the invention to overcome the low inherent stability of certain molecular sieves so that water can be removed therefrom without collapse of the framework structure.
It is a further object of the invention to overcome the low stability of certain molecular sieves by a solid state reaction with a salt.
The low stability of certain molecular sieves during calcination can be overcome by combining the molecular sieve with a salt. This process may be used effectively for zeolites, metal substituted aluminosilicates where the metal is one or more of Zn, Fe, Ga, Ge, Co, Ti, Ni and Mn, and metallosilicates where the metal is one or more of Zn, Fe, Ga, Ge, Co, Ti, Ni and Mn. Thus, the method of the invention provides for the formation of certain molecular sieves that are remain structurally stable during calcination despite water or template removal therefrom.
In a preferred process of the invention, the molecular sieve is mixed with a salt, either directly or as a slurry; and then heated/calcined to remove at least one of water, organics, and adsorbed species. While not wishing to be bound by any theory, it is our belief that the improved stability results from either a solid-state ion-exchange of protons formed on calcination, or compensation for high framework charge within the sieve.